The present invention relates to control devices of inverters for variable-speed control of alternating current (AC) electric motors.
Prior known methods for controlling an inverter of the type that is used for variable-speed drive of induction motors typically include a V/f-constant control technique for controlling an output voltage V1 of the inverter in a way proportional to a primary frequency f1 thereof. With this approach, while it is possible to permit the excitation current of an induction motor to stay substantially constant, application of a load can lead to an increase in voltage potential drop due to the electric motor""s primary resistance r1, resulting in a decrease in induced electromotive force of such motor. This would disadvantageously result in a decrease in magnetic flux of the motor, causing the motor""s output torque to decrease accordingly. Another problem is that the xe2x80x9crealxe2x80x9d velocity tends to decrease with respect to a velocity command, resulting in an increase in velocity variability ratios. One approach proposed to avoid these problems is to employ automatic torque boost control techniques for achievement of torque increase in middle and low velocity regions, in particular.
A typical known approach is to detect an electric motor current by use of separate current detectors corresponding in number to two phases, as provided on the inverter output side; then convert in magnetic flux phases this current from stator coordinates into rotating magnetic field coordinates (uvw/dq conversion) to obtain a torque current Iq; then determine through computation a primary resistance voltage drop component r1xc2x7Iq in accordance with the current obtained; and, thereafter, calculate a voltage command Vm through addition of it to a V/f-constant voltage (induced electromotive force). In this case, the voltage command is Vm=Kxc2x7f1*+r1xc2x7Iq. Here, K is the proportional gain. Such control for permitting an output voltage value to increase with an increase in current in this way is called xe2x80x9cautomatic torque boost control.xe2x80x9d Further, a slip frequency fs in proportion to the torque current can be added to a reference frequency f0* to thereby compute an inverter output frequency command f1*. In this case, f1*=f0*+Ksxc2x7Iq. This control is known as slip compensation among those skilled in the art. These schemes suffer from a cost increase due to the fact that two separate current detectors for independent detection of two phases are required on the inverter output side. On the other hand, another scheme is also available for computing the torque current equivalent components from the average value Idc of inverter DC input currents and then performing the torque boost in accordance therewith. This control method constitutes a scheme for detecting Idc to compute an effective power current Iqx approximating the torque current Iq for compensation of the inverter output voltage in accordance with the significance of this current, in view of the fact that Equation 1 is established due to the equality of power on the DC side of the inverter to that on the AC side thereof.                     "AutoLeftMatch"                                                                              Vdc                  ·                  Idc                                =                                  3                  ⁢                                      xe2x80x83                                    ⁢                                      V1                    ·                    I1                    ·                    cos                                    ⁢                                      xe2x80x83                                    ⁢                  ϕ                                                                                                        =                                  3                  ⁢                                      V1                    ·                    Iqx                                                                                                          (        1        )            
Here in the Equation 1, Vdc is the DC voltage of an inverter, Idc is the DC current average value, V1 represents the significance of an inverter output voltage (phase voltage), I1 represents the significance of an electric motor current, and cosxcfx86 is the power factor. However, in AC electric motors, such as induction motors, the output voltage V1 is to be controlled in a way substantially proportional to the inverter frequency in order to control the value of V/f so that it remains constant. Due to this, V1 decreases in low speed regions, resulting in a noticeable decrease in Idc, as apparent from Equation 1 above. Accordingly, the effective power equivalent current Iqx decreases in detection accuracy, which disadvantageously poses a problem in that the automatic torque boost control decreases in accuracy.
Alternatively, current limit control is designed to detect three phase components of an inverter output current; and, when a current value goes beyond a pre-specified current limit level even with respect to one phase thereof, the inverter output frequency is reduced to thereby lower the slip frequency of an induction electric motor for causing the motor current to stay below the limit level, thus preventing occurrence of unwanted over-current trip phenomena. In this case, at least two motor current sensors are required.
Due to this, several schemes for computation and detection of the inverter""s output current from a DC input current(s) of the inverter to satisfy electric motor sensorless design requirements have been proposed to date, such as for example those as disclosed in Japanese Patent Laid-open Nos. 8-19263 and 6-153526, and IEE Proceedings Vol. 136, No. 4, Jul. 1989 Pages 196-204. Also, some important teachings as to the relation among the inverter""s DC input current and output current plus gate states have been recited in Journal xe2x80x9cDxe2x80x9d of the Institute of Electrical Engineers of Japan, xe2x80x9cSmoothing Capacitor""s Capacitance Reduction and Rapid Stop/Restartup Control Method for Voltage-Type PWM Converters,xe2x80x9d (Apr. 1, 1992) at page 33.
In these electric motor current sensorless techniques, as recited in the above-identified Japanese documents, the one described in Japanese Patent Laid-open No. 8-19263 is designed to sample-hold a DC current in all the gate states once at a time whenever a gate state changes, resulting in an output of a sample-hold circuit varying whenever the gate state changes. Further, a difference (DC current change component) between two sample-hold circuit output values is computed in units of gate states, which requires the use of xe2x80x9cspecialxe2x80x9d A/D converters and microcomputers of the type which may offer high-speed operabilities. As for the technique of Japanese Patent Laid-open No. 6-153526, this Japanese document is completely silent about any practical configuration including the arrangement of sample-hold circuitry and how to make sample-hold signals required. Additionally, the Japanese Patent Laid-open No. 6-153526 teaches inverter output current detection methodology; however, it fails to teach current limit control and automatic torque boost control methods.
The present invention has been made in view of the above problems and its object is to provide a control device for use with an inverter operable to detect from the inverter""s DC input current the amplitude of an electric motor current along with a torque current and/or excitation current thereof, and then using them to perform current limit control, automatic torque boost control, or velocity sensorless vector control and the like.
An inverter device in accordance with the present invention comprises a three-phase inverter which converts DC electric power into AC power as a supplement to an AC electric motor, and a control device operatively associated therewith. The control device includes a phase current detection section for outputting a phase current waveform based on a DC input current, and an arithmetic or operational processing section based on the phase current waveform for performing operational processing tasks for control of the inverter device.
According to the present invention, since a three-phase AC phase current is detected on the basis of the DC input current, it will no longer be necessary to provide a current detector for each phase. Owing to this, the inverter device may be reduced in size or lowered in cost. Further, since the inverter is controlled based on the phase current waveform, it is possible to adequately control the inverter without having to speed up the operational processor unit.
A control device of another inverter device also incorporating the principles of the instant invention is arranged to include sample-hold signal creation means for selecting in units of specified phase periods of inverter output voltage phasesxe2x80x94for example, 60- or 120-degree phase periodsxe2x80x94one gate state from among gate states for causing a positive side arm switching element or a negative side arm switching element of a three-phase inverter to turn on only within a time period corresponding to one phase of three phases thereof, and also from gate states for causing it turn on within periods corresponding to two phases (six kinds of gate states in total since each consists of three kinds), and a sample-hold circuit for sample-holding an inverter DC input current in the one gate state as selected. Whereby, an output of the sample-hold circuit is rendered variable continuously in a way synchronous with the electric motor current within a time period corresponding to the specified phase of the inverter output voltage phases.
A control device of another inverter device in accordance with this invention includes a first sample-hold circuit for sample-holding a DC input current of an inverter in a gate state that causes a positive side arm switching element or negative side arm switching element of a three-phase inverter to turn on within a period corresponding to only one phase while letting it turn off within periods corresponding to the remaining two phases thereof, and a second sample-hold circuit for sample-holding a DC input current in a gate state that causes it turn on within periods corresponding to two phases while turning it off within a period corresponding to the remaining one phase thereof. With such an arrangement also, an output of the sample-hold circuit is made variable continuously in synchronism with the electric motor current within the time period equivalent to the specified phase period of the inverter output voltage phases.
For the purposes of current limit control, the control device of the inverter device in accordance with the present invention is further arranged to reduce the inverter""s output frequency when any one of the output values of the first and second sample-hold circuits goes beyond the preset level. Whereby, the resultant slip frequency of an induction electric motor decreases, thus making it possible to achieve the intended current limit at high speeds.
For the torque boost control, the control device of the inverter device in the present invention is further arranged to provide means for calculating a torque current and/or excitation current of an AC electric motor on the basis of a reference phase with an inverter output frequency command integrated and the first and second sample-hold circuit output values, and means for varying an output frequency or output voltage of the inverter in accordance with a calculated value. Whereby, it is possible to achieve the intended torque increase (automatic torque boost control) by letting the inverter output voltage increase with an increase in torque-component current upon increasing of the load. Furthermore, it is possible to reduce the deviation of a real speed or velocity with respect to a velocity command due to slip compensation for allowing the inverter output frequency to increase with an increase in torque current.